Torrefied cellulosic biomass may be considered a carbon-neutral solid fuel capable of substituting for coal in powdered coal fired combustion units, such as those found in electricity-generating facilities. The carbon neutrality comes about since the carbon dioxide produced by combusting the torrefied cellulosic biomass is in turn consumed by the photosynthesis of carbon dioxide and water to form cellulose, thus closing the carbon cycle by utilizing nature's own resources.
Torrefaction is a process whereby cellulosic biomass is heated in a diminished oxygen environment at temperatures of from 240° C. to 300° C. resulting in water loss and causing volatile organic compounds to evaporate from the biomass. After torrefaction, the original moist biomass is converted to a brittle, brown, low-density solid with a high energy content per unit mass. However, to be economically viable as a coal substitute, the torrefied biomass must have its bulk density dramatically increased so as to facilitate storage and shipment, and keep associated costs, including transportation, to a minimum.
Procedures for compacting torrefied biomass are mentioned in the prior art but rarely discussed in detail. In GB 2448531, torrefied biomass is compacted and presented in pelletized form involving a process using an oil-based binder additive. The binder is necessary to address the brittleness and friability that would otherwise be exhibited by the pellet. The use of an extrinsic binder when preparing compacted torrefied biomass capable of serving as a coal fuel substitute is not desirable, as it provides the energy-generating industry with additional environmental issues the industry is striving to avoid. Obstacles associated with making compacted torrefied biomass that is prepared in the absence of an extrinsic binder additive include the fragile nature of the biomass, as it typically breaks and disintegrates when subject to physical mechanical forces. Other obstacles that must be addressed include a high degree of water uptake, which is an undesirable property, as it frequently leads to disintegration of the compacted biomass, or pellet. Consequentially, there exists a need to provide methods and systems for making compacted torrefied biomass that does not exhibit these undesirable properties and that can be made without an extrinsic binder additive. These methods/systems also should be capable of making biomass products having improved physical attributes.